I obvious answer is "it's up to you" i guess.
If it were me doing my own battery then I'd want to set 0 and 100% to the true values, then when i looked i would expect to see it within the range 20-90% and know everything was operating as it should.
If i was setting it up for someone else's battery, then i might take a view on their battery knowledge and if they would be expecting to charge to 100%, and be able to access power down to 0%, in which case they might get confused if it cuts off when still showing 20% and won't charge above 90%, then I'd calibrate the indicator to read 0-100 over the 20-90% range.